As fewer hydrocarbon resources are available and global demand continues to increase, methods and devices to produce hydrocarbons efficiently are becoming increasingly crucial.
One method of increasing efficiency and reducing the cost of producing hydrocarbons is to drill a single wellbore that intercepts many zones. Once the well is drilled it may be necessary to stimulate each zone independently. Typically the stimulation process begins nearest the lower end of the well otherwise known as the toe of the well.
In the past, the process began by drilling a well, during which, the number of formations that are to be stimulated is determined, keeping in mind the upper limit that can be run into a wellbore.
In the past, systems have been used that may have for example, 21 different stages. In turn, each stage needs a different ball size. Typically the lowermost stage will use the smallest ball size and each stage will use progressively larger ball sizes as the stimulation process moves from the toe of the well towards the surface.
When running the twenty one zone system into the formation the various sliding sleeves and zone isolation packers are assembled on the surface, starting with the smallest sliding sleeve at the bottom so that the smallest ball will activate the smallest or lowermost sleeve.
The production tubing is assembled on the surface. At the lowermost end of the tubing may be a fill shoe or it may have a pressure actuated sliding sleeve or toe sleeve. The toe sleeve is typically opened with tubing pressure alone and a ball is not necessary to actuate the sliding sleeve in the toe sleeve. At various intervals along the production assembly, zone isolation devices and corresponding sliding sleeve assemblies may be placed.
Zone isolation may be accomplished by cementing the production tubing and sliding sleeve system into place. Other devices may be used for formation zone isolation such as wellbore packers, including swellable packers, hydraulic control line packers, and mechanically actuated packers.
The zone isolation devices are located along the production assembly both above and below each sliding sleeve corresponding with each formation zone that is going to fraced or produced. Typically a ball actuated sliding sleeve is placed so that it is centrally located in a formation zone. Zone isolation devices are placed so that the production tubing is sealed to the wellbore below the formation zone and above the formation zone. Additionally it may be necessary to place anchoring devices at intervals along the length of the production tubular to prevent movement of the production tubular. Any movement of the production tubular could cause the zone isolation devices to shift so that they are no longer located above and below a formation zone or movement could cause erosion of the isolation packer's seal thereby causing the seal to fail.
Each of the sliding sleeve assemblies starting just above the toe sleeve and moving towards the surface utilizes a successively larger ball.
As the production tubular is assembled it is lowered into the wellbore. In those cases where a fill shoe is used the production tubing may be lowered at any rate that keeps the production tubing at least partially filled in order to reduce the buoyancy of the production tubular. In other instances the toe sleeve may be used to seal the lower end of the production tubular. When the lower end of the production tubular is sealed, mud or other fluid may be pumped into the production tubular from the surface. When the mud or other fluid is pumped into the production tubular from the surface the buoyancy of the production tubular may be controlled. By controlling the production tubular's buoyancy the production tubular may be floated into any relatively long horizontal sections of the wellbore.
Practical issues related to the size of the larger and smaller balls tend to limit the number of sleeves in a system. While referring generally to a ball to engage each seat in the corresponding sliding sleeve, any object such as a dart or plug, that can move through the well and engage the seat in the sliding sleeve may be used.